Extraction and treatment of heavy metals

ABSTRACT

Contaminated waste streams or soil or sludge is subjected to a first treatment stage with an acid liquor at a pH below 2 and at a solids content from 5 to 30%; then the liquid and solid phases from the first treatment stage are separated; the solids from the first treatment stage are mixed with a fresh acid liquor at a pH below 2 and a solids content from 5 to 30%; the liquid and solid phases from the second treatment stage are separated; the liquor separated from the first treatment stage is reacted with a base to precipitate the metals; the precipitated metals are separated and the liquor is recycled for reuse in the process; the solids separated from the second treatment stage are neutralized to adjust the pH to a level acceptable for a soil conditioner or fertilizer or other customized products; and the liquor from the second stage is used as the acidic liquor in the first treatment stage.

This invention relates to the removal of heavy metals from sewage sludgeand contaminated soils to produce a soil enhancer and fertilizer.

BACKGROUND OF THE INVENTION

Sewage sludge is usually discarded at the bottom of settling ponds. Oldsludges are of little value in terms of the useful organic content, asthey have been biodegraded and may contain high levels of heavy metals.They are usually the anaerobically digested or aerated end products ofwaste water treatment. These sludges often have a useful nutrientcontent of phosphate and nitrates. New sludges still have a high organiccontent and generate sulfur gases in particular hydrogen sulfide.

U.S. Pat. Nos. 3,942,970 and 5,387,257 propose the use of sewage sludgesin fertilizer.

However most governments set maximum acceptable levels for metal contentin fertilizers and soil enhancers particularly for arsenic, cadmium,chromium, copper, mercury, molybdenum, nickel, lead, selenium and zincbecause of the potential that these metals will enter the food chain.

U.S. Pat. No. 5,009,793 discloses the removal of heavy metals fromsludges and contaminated soils.

U.S. Pat. No. 5,178,776 discloses the removal of metals from sludges byacidification followed by a heating step to sterilize the treatedsolids.

U.S. Pat. No. 5,051,191 treats non oxidized sewage sludge at a lowsolids content to remove heavy metals by acid solubilization andsubsequent precipitation from the liquid phase to leave a decontaminatedsludge that is neutralized and then used as a soil conditioner andfertilizer.

USA application 2002/0153307 improves on this process by operating at ahigher acid pH to reduce the solubilization of fertilizing elements inthe sludge.

A problem with prior art processes is that a waste stream is stillproduced. Also because the value of the decontaminated sludge is nothigh the cost of carrying out the process must be kept to a minimum.Some prior art processes use expensive chemicals and require heating ofthe sludge all of which adds to processing costs.

It is an object of this invention to provide a process that overcomesthe problems of the prior art.

BRIEF DESCRIPTION OF THE INVENTION

To this end the present invention provides a method of decontaminatingheavy metal contaminated soil or sludge which includes

-   -   a) treating soil or sludge with an acid liqour at a pH below 2        and at a solids content from 5 to 30% in a first treatment stage    -   b) separating the liquid and solid phases from the first        treatment stage    -   c) mixing the solids from the first treatment stage with a fresh        acid liquor at a pH below 2 and a solids content from 5 to 30%        in a second treatment stage    -   d) separating the liquid and solid phases from the second        treatment stage    -   e) treating the liquor separated in step b) from the first        treatment stage to precipitate metals    -   f) separating the precipitated metals from the liquor of step e)        and recycling the liquor for use in the process    -   g) treating the solids from step d) to adjust the pH to a level        acceptable for a soil conditioner or fertilizer    -   h) using the liquor from step d) as the acidic liquor in the        first treatment stage for fresh batches of soil or sludge.

The method of this invention produces a solid heavy metal mix which maybe further treated to produce metals for use in other processes. Noliquid effluent is produced as all liquid is reused in the process. Theliquor from step f) is usually mixed with fresh acid to ensure the pH isbelow 2. The low pH in the treatment stage has the additional benefit ofdestroying more than 99% of any pathogens present in the sludge, whichis most likely with new sludges.

The acid used may be any acid suitable for dissolving the metals and maybe selected from phosphoric, nitric, citric, acetic or sulfuric acid.Sulfuric acid produced as an industrial by product is preferred becauseit is inexpensive.

The acid solution concentration is generally from 0.2% to 10% preferably1% and is usually used in a ratio of up to 10 parts by weight of acidsolution to one part by weight of sludge on a dry weight basis,depending on the degree of heavy metal contamination. Preferably therange is between three and six parts of acid solution to one part byweight of sludge.

This double counterflow process ensures that the metal content of mostsludges or contaminated soils is reduced to acceptable levels.

Newer sludges are generally lower in metal contamination levels and asingle treatment stage may be sufficient. However, because newer sludgesmay include flocculants used in the sewage treatment plant, they mayhave to be treated at lower solids content than older sludges. If thecontamination is particularly high as in older sludges an additionalthird treatment stage may be needed.

The residence time in the reaction vessel may be from 10 minutes to 12hours and depends in part on the time to fill and empty the vessel. Themixture in the vessel is preferably agitated to ensure thorough mixingof the sludge and acid liquor. New sludges often have a higher organiccontent and offensive odours due to the presence of hydrogen sulfide.For treating newer sludges it is preferred to use a closed reactionvessel to contain the hydrogen sulfide and other gases generated duringthe dissolution step. The head space in the closed reaction vessel maycontain an ozone generation system which converts the hydrogen sulfideto sulfurous and sulfuric acid which dissolve in the liquid phase. Theozone generation system may be any conventional ozone generation systembut is preferably a bank of ultraviolet light lamps emitting wavelengthsthat convert oxygen into ozone. Alternativley a ozone may be suppliedinto the head space from an external generator or supply. Where only oldsludges are to be treated an open vessel is feasible.

Where both old and new sludges or contaminated soils are available itmay be viable to blend the sludges and soil and treat the blend. Thisreduces the level of contamination compared to old sludges and reducesthe amount of hydrogen sulfide compared to the newer sludges. Because insome locations both old and new sludges are available it is preferred toblend sludges and use an optimum plant layout of two closed vessels.

The two separation stages b) and d) may use any suitable solids liquidseparation technique including a filter press or centrifuge.

The metals are precipitated by adding to the twice used liquor from thefirst treatment stage a base such as potassium hydroxide to bring the pHto a value of about 7. Potassium is preferred as the base because it isa plant nutrient. The precipitated metals are separated by any suitablephysical separation means including filtration through a filter press orby way of a centrifuge. In order to facilitate extraction of the metalssequestering and complexing agents may be added to the solution at anystage of the process at a concentration of 0.5% to 10% by weight andpreferably are added in the first treatment stage. These agents includeacetodiphosphonic acid, aminotris (methylene phosphonic acid),(2hydroxyethyl) iminobis-(methylene phosphonic acid),ethylenediaminetetrakis-(methylene phosphonic acid),hexamethylenediaminetetrakis-(methylene phosphonic acid),diethylenetriaminepentakis-(methylene phosphonic acid), ethylene diaminetetracetic acid and its salts.

The precipitate usually contains an iron-aluminum-phosphate hydroxide inwhich the heavy metals are scavenged. By careful control of the pH itmay be possible in some cases to separate the metals from the phosphateso that the phosphate can be returned to the decontaminated sludge. Theextracted metals may be immobilized by encapsulation as constructionfill such as dolocrete. In some cases extraction of particular heavymetals by smelting may be economically feasible. The solids recoveredfrom step d) are generally too acidic and are neutralized by mixing with5% to 75% by weight of pulverized limestone on a dry weight basis. Thisproduct is acceptable as a soil amendment material and may be blendedwith other materials such as phosphate or nitrogen fertilizer to producea range of materials such as soil conditioner, compost, or fertilizer.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is described with reference tothe drawing FIG. 1 which is a schematic illustration of the plant andprocess of this invention. The plant is intended for use in treatingboth old and new sludges and contaminated soils and some waste streams.

The plant consists of two closed reaction vessels 11 and 13 a heavymetal precipitation tank 16 and centrifugal decanter separators 12, 14and 17 and a solids blending vessel 15.

Old sludge from a waste treatment deposit and new sludge from atreatment plant are blended and introduced via line 20 into the reactionvessel 11. A sulfuric acid containing solution of pH 1 is introduced vialine 25 into the tank. The solution at a solids content of about 25% isagitated for a time of from half an hour to 2 hours. Sequestering agentsmay be added at this stage to facilitate the separation of heavy metals.The slurry is then passed via line 21 to the centrifuge 12 where the 5solids are separated and passed via line 22 to the second treatmentvessel 13. The treatment conditions in 13 are the same as for vessel 11.

Because the blended sludge generates hydrogen sulfide during the acidtreatment, the vessels 11 and 13 are closed during the filling andmixing stage and the ultra violet ozone generators 31, located in thehead space of the vessels, are activated to oxidize the hydrogen sulfideto water soluble sulfurous and sulfuric acids. An alternative toproviding the ozone generator in the head space is to install anexternal ozone generator next to the vessels 11 and 13 and pipe theozone into the head space. This avoids the need to periodically cleanthe lamps or generators within the head space and also avoids corrosionof the lamps or generators by the acidic environment in the head spaceof vessels 11 and 13. Another alternative is to pump off the gas fromthe head space to a gas scrubber to remove the noxious gases. It ispreferred to introduce ozone into the headspace so that the sulfur gasesare converted to sulfuric acid and thus enrich the acidity of therecycled treatment liquor.

The liquor separated in the centrifuge 12 is passed via line 26 to theheavy metal holding and precipitation tank 16 where potassium hydroxideis added to adjust the pH of the solution and precipitate the metals asmetal salts.

The slurry from the precipitation tank 16 is then passed via line 27 tothe centrifugal separator 17 where the metals are extracted as a residue28 and the liquor is recycled via line 29 to the reaction vessel 13.Fresh sulfuric acid and make up water may be added to line 29 to ensurethe acidity of the solution added to reaction vessel 13 is below pH 1.Additional make up water and acid which may be plant waste water isintroduced into the treatment vessels 11 and 13 via line 30. The residue28 may be sold as a zinc matte for recycling or encapsulated for use asconstruction fill.

The treated slurry exits reaction chamber 13 via line 23 to thecentrifugal separator 14. The liquor separated at 14 is passed via line25 to the first reaction vessel 11.

The decontaminated solids 24 exiting the separator 14 are passed to theneutralization mixer 15 where they are blended with crushed lime stoneto achieve a desirable soil pH. This product may be further blended in ablending installation 32, to provide a range of commercially acceptablesoil conditioners and fertilizers 33.

From the above description those skilled in the art will realize thatthe present invention provides an environmentally sound method oftreating sewage sludges or contaminated soils without any offensiveodours and any liquid effluent stream. In addition these advantages areprovided using low cost reagents such as sulfuric acid, potassiumhydroxide and crushed limestone that are usually available as industrialwaste or by products.

Those skilled in the art will also realize that the present inventioncan be implemented in a variety of embodiments without departing fromthe core teachings of the invention.

1. A method of decontaminating heavy metal contaminated soil or sludgewhich includes a) treating soil or sludge with an acid liqour at a pHbelow 2 and at a solids content from 5 to 30% in a first treatment stageb) separating the liquid and solid phases from the first treatment stagec) mixing the solids from the first treatment stage with a fresh acidliquor at a pH below 2 and a solids content from 5 to 30% in a secondtreatment stage d) separating the liquid and solid phases from thesecond treatment stage e) treating the liquor separated in step b) fromthe first treatment stage to precipitate heavy metals f) separating theprecipitated heavy metals from the liquor of step e) and recycling theliquor for use in the process g) treating the solids from step d) toadjust the pH to a level acceptable for a soil conditioner or fertilizerh) using the liquor from step d) as the acidic liquor in the firsttreatment stage for fresh batches of soil or sludge.
 2. A method asclaimed in claim 1 in which the acid is sulfuric acid.
 3. A method asclaimed in claim 1 in which the heavy metals are precipitated by addinga base to adjust the pH of the liquor to precipitate the metals as salts4. A method as claimed in claim 3 in which the base is potassiumhydroxide.
 5. A method as claimed in claim 1 in which the solids fromstep d) are blended with crushed limestone.
 6. A method as claimed inclaim 1 in which the first and second treatment stages are carried outin closed vessels containing a source of ozone in the head space of theclosed vessels.
 7. A method of decontaminating contaminated soil orsludge which includes sulfur containing materials which method includesa) treating soil or sludge with an acid liquor at a pH below 2 and at asolids content from 5 to 30% in a closed vessel containing a source ofozone in the head space of the closed vessel. b) separating the liquidand solid phases from the treatment stage c) treating the liquorseparated in step b) from the treatment stage to precipitate heavymetals d) separating the precipitated metals from the liquor of step c)and recycling the liquor for reuse in the process e) treating the solidsfrom step b) to adjust the pH to a level acceptable for a soilconditioner or fertilizer
 8. A method as claimed in claim 7 wherein theozone is externally generated and introduced into the head space.